The present invention relates generally to disc pack assembly in a disc drive, and more particularly to eccentricity correction of a number of discs mounted on a spindle motor such that vibration of the discs can be reduced during the read/write operation.
A disc stack assembly usually has one or more discs mounted through the hub of a spindle motor. The spindle motor rotates the discs about an axis of rotation through the center of the hub at high speed to facilitate fast data retrieval and storage. Each disc has a number of circular tracks for storing data. A sensor, positioned above the tracks, moves radially across the tracks on a disc to retrieve or store data.
Any unbalance in the discs stack will induce vibrations in both planar and axial directions causing error in the sensor when reading and writing of data on the tracks. As the sensor is in close proximity to the disc surface, such vibrations may also result in the sensor crashing into the disc surface causing damage to the disc surface. In order to avoid such problems, various methods have been adopted. U.S. Pat. No. 4,683,505 issued on Jul. 28, 1987 to IBM for xe2x80x9cAlternately Centered Disk Pack Assembly and Methodxe2x80x9d describes the use of alternately diametrically biasing discs in a disc stack assembly about the spindle axis of rotation. The discs are positioned as a function of outer edges such that alternated opposite outer edges of the discs line up as though they were the outer edges of centered nominal diameter discs.
Other prior art teaches the balancing method where the unbalance of the disc stack is measured and a mass of known weight is added to the disc stack to reduce the unbalance. U.S. Pat. No. 5,130,870 issued on Jul. 14, 1992 to Seagate Technology, Inc. is one such patent which teaches the use of epoxy adhesives and filler material of known density attached to the disc clamp to reduce unbalance in a disc pack. Although these prior art methods were able to reduce the planar vibrations by minimizing the unbalance contributed by the discs, the considerations in these methods does not include the contributing factor of unbalance in the spindle motor. Therefore, there remains a need for an improved method of reducing the unbalance in a disc pack where the unbalance of the spindle motor is taken into consideration together with the discs and the way in which they are mounted on the hub of the spindle motor.
It will be evident from the following description that the present invention offers this and other advantages in minimizing the unbalance forces and vibrations in a disc pack.
According to one aspect of the invention, there is provided a method to reduce unbalance in a disc pack. The disc pack having a spindle motor with a hub, a disc, a spacer and a disc clamp is assembled by the method such that the unbalance is minimized by incorporating a quantified unbalance of the spindle motor in the biasing process. The method comprises steps of creating a known unbalance on the spindle motor; and biasing the disc and the spacer to achieve an allowable unbalance with respect to the unbalance of the spindle motor. The step of creating the known unbalance further comprises the steps of demarcating the unbalance on the spindle motor; and determining the direction of the unbalance of the spindle motor. And the step of biasing the discs involve the optimization of the unbalance direction of the disc, spacer and the spindle motor using vector sum of each three components such that the resultant is within an allowable unbalance. In the method, the spindle motor has a feature to provide the orientation identification of the unbalance in the motor. The feature is used to locate the motor hub at a specific orientation during the biasing process.
In one embodiment of the invention, the feature is a hole in the spindle motor to provide a known unbalance.
In another embodiment of the invention, the feature is a marker on the spindle motor.
The allowable unbalance falls in the range of 30 mg.cmxc2x115 mg.cm.
Another aspect of the invention provides a disc drive having a disc pack, the disc pack comprising of a spindle motor with a hub, at least one disc, at least one spacer; and a disc clamp. The spindle motor has a known unbalance incorporated into a biasing process to minimize the unbalance of the disc pack. The known unbalance is provided by drilling a hole in the spindle motor and demarcated with a feature to provide orientation identification. The feature locates the hub in a specific orientation during the biasing process. The known unbalance of the spindle motor falls directly opposite the feature within a range of xc2x130xc2x0 about a line through the center of the hub and the feature.
In one embodiment of the invention, the feature is an indentation on the spindle motor.
The present invention provides a disc drive having a disc pack, the disc pack comprising a disc, a spacer and a spindle motor assembled by biasing the disc and spacer in directions determined by means of a vector sum equation of the unbalance of the spindle motor, spacer and disc.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.